1. Field of the Invention
This invention relates to a circuit for discriminating among pulses in a pulse sequence in accordance with the time interval between adjacent pulses to produce timing pulses of substantially constant width irrespective of the frequency of the pulses in the pulse sequence. More particularly, the invention relates to a circuit producing accurate timing pulses in a monopulse servo system employed in a disc pack data storage apparatus.
2. Description of the Prior Art
Disc pack data storage systems typically include at least one surface on which servo data are prerecorded and are used to generate timing information and position information for other parts of the system.
Among the techniques for recording the servo data is a system known as the dipulse system. The dipulse system is exemplified by U.S. Pat. No. 3,534,344 to Santana, which discloses servo tracks having odd dipulses alternating with servo tracks having even dipulses. Servo read circuitry is employed to extract timing and positioning information from the servo tracks.
Another servo system, exemplified by U.S. Pat. No. 3,691,543 to Mueller employs tripulses and circuitry for extracting timing and position information therefrom.
Still another system, with which the present invention is particularly useful, is known as the monopulse system. In such system there are alternating odd and even servo tracks, each track being composed of a series of frames. Each such frame includes a pair of sync pulses that are spaced from one another by a prescribed relatively short interval; each frame also includes one or more position pulses spaced from the sync pulses and from each other by a second relatively long interval. The sync pulses on adjacent tracks are spaced in substantial radial alignment and distinction between odd and even position pulses is made by measuring the time of occurrence of the position pulses with respect to the sync pulses. For accurate timing and positioning it is thus important to produce timing pulses of uniform duration for all tracks and for all relative speeds between the tracks and the servo head.
A prior art system for producing timing pulses in a monopulse servo system employs a monostable multivibrator which produces a timing pulse of fixed duration. Such prior art system generates accurate timing information when the disc speed is constant. When the disc speed varies, however, production of accurate timing information suffers. Moreover, such prior art system is sensitive to noise.